Styrene emulsion polymerization process



I Patented Nov. 15, 1949 STYRENE EMULSIOlW POLYMERIZATION PROCESS Harold F. Park, East Longmeadow, and Costas H. Basdekis and Ralph I. Dunlap, In, Springfield, Mass, assignors to Monsanto Chemical Company, St. Louis, Mo., a corporation oi Delaware No Drawing. Application December 5, 1946, Serial No. 714,137

2 Claims. (Cl. 260-935) This invention relates to polystyrene. More particularly, the invention relates to a process for the emulsion polymerization of monomeric styrene.

The polymerization of styrene in aqueous emulsion, though long known, has been beset with many difiiculties. Long washing procedures have been necessary to produce a clear, homogeneous polymer. The necessary addition of coagulants to the emulsions has increased the cost of the process and the coagulated particles have been diflicult to wash and dry. Further, the coagulated particles have been dimcult to handle in molding operations and the molded pieces produced therefrom have been relatively poor from the standpoint of strength, clarity, heat resistance and ageing resistance.

It is an object of this invention to provide an improved process for the emulsion polymerization of styrene.

A further object is to provide, by an emulsion process, polystyrene having improved clarity,

strength, and heat and ageing resistance.

These and other objects are attained by polymerizing a mixture of monomeric styrene, emulsifying agent, modifying agents, catalyst and buffer under specific operating conditions and using crit cal ranges of the various-ingredients.

The following example is given in illustration and is not intended as a limitation on the scope of this invention. Where parts are mentioned, they are parts by weight.

Example The following three solutions were made up separately:

Potassium persulfate pheric pressure for about 10 minutes to expel disminutes. Throughout the entire procedure, the

solved and occluded air. Solution C was divided into five approximately equal portions and one portion added to solution B at the end or the initial reflux period. Thereafter, solution A was added to solution B slowly over a period of about one hour. The remaining portions of solution C were added to solution B at approximately fifteen minute intervals during the addition of solution A. After all of the ingredients had been incorporated, the reaction was continued for about 15 reaction medium was heated at reflux temperature at atmospheric pressure, i. e., from about C. to about C. v

The contents of the reaction vessel were continuously agitated during the polymerization process, the agitation being controlled to prevent turbulent movement of the liquids. During the polymerization reaction, the pH of the medium varied between about 10 and about 11. At the end of the process, a stable, aqueous emulsion of polymerized styrene was obtained which was then dried directly in a drum drier. It was not neces-- sary to coagulate the mulsion prior to the drying step. The polystyrene obtained was in theiorm of flakes which could be easily molded under heat and pressure to yield articles which were clear and free from haze.

The particular formulation used, the method of forming the emulsion and the polymerization process present many critical phases which must be accurately controlled to obtain a polymerized product having good moldability and physical properties.

One of the least critical aspects is the ratio of styrene to water which may be variedfrom 1:1 to 1:10. The final emulsion may be further diluted with water if desired.

The emulsifying agents which may be used are soaps, quaternary ammonium salts, and other well known emulsifying agents which are eflicient in an alkaline medium having a pH of between 8 and 11. Examples of the emulsifying agents which may be used are the alkali metal salts of long chain fatty acids such as lauric acid, palmitic acid, stearic acid, oleic acid, coconut oil fatty acids, polyacrylic acids, polymerized alpha rate, etc.

assasos alkyl acrylic acids, the alkali metal salts of styrene-maleic anhydride copolymers, octadecyl amine acetate, quaternary ammonium salts such agents may be used. The amount of emulsifying agent used is restricted to a range of from about 0.05 part to about 2.0 parts per 100 parts of monomeric styrene. If more than 2.0 parts of emulsifying agent is used, the resulting emulsion must be coagulated and the coagulant washed out to obtain a clear polymer stable to heat and ageing. It has further been found particularly advantageous to divide the emulsifying agent between the water and the styrene monomer prior to starting the polymerization. This may be varied by using as little as up to about 60% of the emulsifying agent in the water with the remainder being-incorporated in the styrene monomer.

The modifying agent shown in the example, dodecyl mercaptan, may be substituted, in whole or in part, by other mercaptans having from 4 to 20 carbon atoms in the alkyl chain, e. g., decyl mercaptan, lauryl mercaptan, butyl mercaptan, amyl mercaptan, tertiary-butyl mercaptan, tertiary-amplmercaptan, tertiary-dodecyl mercaptan, octyl-dodecyl mercaptan, tertiary-hexyl mercaptan, tertiary-heptyl mercaptan, tertiaryoctyl mercaptan, tertiary-tetradecyl mercaptan, tertiary-hexadecyl mercaptan, etc. Mixtures of the mercaptans may be used. The amount of modifying agent may be varied according to properties desired in the final product. For commercial molding powders the preferred range is from about 0.025 to about 0.35 part per 100 parts of styrene monomer. The modifier may be added to the styrene monomer or to the water prior to the start of the polymerization reaction. It may also be divided between the water and the styrene. Further, it may be added to the polymerization reaction gradually during the course of the polymerization or step-wise during polymerization.

The potassium persulfate used in the example may be replaced by certain other well-known catalysts including sodium perborate, hydrogen peroxide, acetyl peroxide, and other catalysts which are soluble in water and have an oxidation reduction potential of less than -1.5. Such wellknown catalysts as benzoyl peroxide, lauroyl peroxide, di(tertiary butyl) peroxide, ,etc., are, for practical purposes, insoluble in water and cannot be used even though their oxidation reduction potential is less than -1.5 since their use in the present process produces polymers greatly inferior to those made with the preferred catalysts. The catalyst is preferably dissolved in water together with the buffer, but separately from the monomeric styrene and emulsifying agent. The catalyst solution is then added to the polymerizing medium, either continuously or at intervals during the addition of styrene monomer. It can also be added directly to the'water solution of emulsifying ent prior to addition of styrene monomer.

Sodium hydroxide is used in the example as a medium for maintaining the pH of the emulsion on the alkaline side, i. e., between 8.5 and 11. Other water-solublealkaline materials such as hydroxides of alkali metals and alkaline earth metals, quaternary ammonium compounds, etc. may be used to replace the sodium hydroxide in part or in whole. The nature and amount of alkaline material used will depend somewhat on A mixture of two or more emulsifying the catalyst used, especially on the acidity of the decomposition products of said catalyst. However, catalyst and alkaline material must be so chosen that the amount of the latter needed will be less than about 0.5 part per 100 parts of styrene and the amount may be as small as 0.002

. part if a catalyst such as hydrogen peroxide is used. If the polymerization is carried out in the absence of air, e. g., under a carbon dioxide atmosphere, the lower range of alkaline materials may be used.

The temperature of the polymerization process of this invention may be varied between about 60 C. and the temperature of reflux at atmospheric pressure. However, the speed of the reaction is considerably greater at the reflux tem-, perature and the product obtained is not adversely affected by the. use of said temperature, so that the preferred process is to use the reflux temperature at atmospheric pressure which is' from about C. to about C. The time necessary to complete the polymerization by the process of this invention will vary with the temperature and with the rate of addition of the various ingredients. Generally. the polymerization may be completed in from about hour to about 5 hours after all ingredients have been incorporated.

It has been found that the amount of agitation imparted to the polymerizing mixture is extremely critical for producing an emulsion of the polymer which may be direct dried. The agitation must be fast enough to completely mix all ingredients, but must be slow enough to prevent the development of surface turbulence. For example, an anchor stirrerfrevolving at 60 R. P. M. in a 100-gallon glass lined autoclave yields a smooth emulsion. Increasing the R. P. M. to 300 causes coagulation of the polymer into a viscous paste which is difficult to disintegrate and dry. If larger vessels and vessels of different shapes are used, the type of agitating blade and speed of agitation must be varied to obtain optimum conditions. But, in any case, the agitation must be slow enough to prevent surface turbulence and yet must cause a thorough intermingling of the ingredients.

The polymer obtained by the process of this investigation contains less than 1% and generally less than 0.5% of the impurities known as methanol solubles which contribute materially to poor heat and weathering resistance. The molecular weight of the polymer is equivalent to that of the best polymers made for molding processes by the mass polymerization methods. Products molded from the polymer have higher impact strength and show a greater deflection than products molded from a'comparable mass polymer. In the table is set forth the physical properties of moldings made from a polymer produced by theemul- The process of this invention provides an ex-- tremely rapid method for obtaining an emulsion emulsion polymers of styrene which can only be molded under extreme conditions and which even then tend to yield poor moldings having flow lines, check marks, poor weld lines, etc.,

the polymers made by the process of this inven-' tion may be molded easily at normal molding temperatures to give clear moldings free from imperfections.

Conventional additives such as dyes, pigments, lubricants and fillers, etc. may be added to the polymer prior to molding. If the additives used do not afiect the polymerization process, they may be dispersed in the monomer before polymerization.

It is obvious that many variations may be made in the process and products of this invention without departing from the spirit and scope thereof as defined in the appended claims.

What is claimed is: I

1. The process which comprises polymerizing 100 parts of styrene in aqueous emulsion in conjunction with (I) from 0.005 to 0.4 part of potassium persulfate, (2) from 0.01 to 0.4 part of sodium stearate, (3) from 0.003 to 0.5 part of sodium hydroxide and, (4) from 0.025 to 0.35 part of dodecyl mercaptan, said process including the steps of dividing the sodium stearate between the water and the styrene monomer prior to mixing the same, maintaining the water at reflux temperature at atmospheric pressure while gradually adding the styrene monomer to the water, adding a water solution of the sodium hydroxide and potassium persulfate in small in-- crements to the refluxing mixture, agitating the refluxing mixture throughout said steps at a rate insuflicient to cause surface turbulence, and continuing the heating at reflux temperature after the flnal addition of the monomer to complete the polymerization.

2. A process which comprises polymerizing parts of monomeric styrene in conjunction with I Part Dodecyl mercaptan 0.25 Sodium stearate 0.4 I Potassium'persulfate 0.1 Sodium hydroxide 0.07

said process including the steps of dividing the sodium stearate equally between the monomeric styrene and the water prior to mixing the same, heating the water solution of the sodium stearate at refluxing temperature at atmospheric pressure for 10 minutes, adding the monomeric styrene containing the sodium stearate slowly to the refluxing solution over a period of one hour while maintaining the solution at reflux temperature, dissolving the potassium persulfate and sodium hydroxide in water, and adding the solution thus formed in five successive portions to the water solution at intervals of fifteen minutes, agitating the refluxing mixture throughout the process at a rate insuflicient to cause surface turbulence of the polymerizing medium, continuing the heating at reflux temperature and atmospheric pressure for fifteen minutes after the final addition of said ingredients, and recovering the polymerized styrene from the emulsions thus produced by direct drying. v

' I HAROLD F. PARK.

COSTAS H. BASDEKIS. RALPH I. DUNLAP, JR.

REFERENCES CITED The following references are of record in the file of this'patent:

Lange: Handbook of Chemistry," pp. 915-916 (1941). 

